The invention relates to an electroluminescent display device comprising a layer of electroluminescent material with an active layer of an organic material, which layer is present between a first and a second pattern of electrodes, which patterns define pixels having a different surface area, at least one of the two patterns being transparent to light to be emitted through the active layer, and a first pattern comprising a material which is suitable for injecting charge carriers by means of a bias current for emitting, the display device comprising drive means for adjusting the bias current of a pixel.
Electroluminescent (EL) display devices may be used in, for example, displays and indicator lamps. An increasing number of organic materials such as, for example, semiconducting organic polymers is used for the active layer in such structures. This increases the number of possible materials for use in these types of display devices. The active layer and the two electrode layers (the electroluminescent display device) preferably comprise a plurality of LEDs, for example, in the form of light-emitting surfaces arranged as segments or matrices, as intended for a display device described in, for example, WO 96/36959 (PHN 15.320), or combinations thereof.
The operation is based on the recombinations of electron hole pairs which are injected into the semiconductor material (during use in the forward direction) from electrodes situated on both sides of the active layer. Due to these recombinations, energy is released in the form of (visible) light, a phenomenon referred to as electroluminescence. The wavelength and hence the color of the emitted light is also determined by the bandgap of the (semiconductor) material.
Notably when using these types of display devices with pixels having a different area, problems arise in realizing the desired brightness at a given signal. The input signal is generally used for controlling a current source which generates a current through the LED (the pixel). The brightness (luminance) of such a pixel is, however, dependent on the density of the current through such a pixel. When using the same current through LEDs with a different surface area, a difference in surface area leads to a difference in the current density and hence to a difference in luminance.
It is, inter alia, an object of the present invention to obviate one or more of the above-mentioned drawbacks.
To this end, a luminescent display device according to the invention is characterized in that the drive means comprise means for varying the current density of the bias current in dependence upon a surface area of a pixel.
The invention is based on the recognition that different electrical parameters (capacitance, current density) are dependent on the surface area of a pixel and may therefore be used as feedback parameters for adjusting the correct bias current.
A preferred embodiment of a luminescent display device according to the invention is therefore characterized in that the drive means comprise means for defining the capacitance of a pixel.
This may be realized in a simple manner by means of a (small-signal) alternating current. A first embodiment is therefore characterized in that the means for defining the capacitance of a pixel comprise means for adding a (small-signal) alternating current to the bias current of the pixel and for measuring the associated (small-signal) alternating voltage.
In addition, the capacitance of a pixel may be defined by means of, for example, a sample-and-hold method, in which a pixel (segment) is supplied with a fixed measuring current and the voltage caused by the measuring current across the pixel is fixed. The measuring current is preferably supplied within a measuring period in which the voltage across the pixel remains limited to a value below the threshold value of the pixel.
The means for defining the capacitance of a pixel may alternatively comprise means for applying a voltage pulse across a pixel and for defining the decay time of the current through the pixel. The measured decay time is then compared, for example, with the decay time of a reference circuit.
Another possibility of defining the capacitance of a pixel makes use of the resonance frequency of a circuit of which the pixel forms part.
Another embodiment of a luminescent display device according to the invention makes use of current measurement. This embodiment is characterized in that the electroluminescent display device comprises at least two pixels having a different surface area, and drive unit means for applying a voltage in the reverse direction across the pixels, and means for defining the reverse current. This embodiment is notably, but not exclusively, suitable for a luminescent display device driven in a multiplex mode.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.